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1 Analysis of Parameters Affecting Recovery Factor in Retrograde Gas Condensate Reservoirs Mohammad Riyahin *, Jahangir Talebi * (Department of Petroleum Engineering, Firoozabad Branch, Islamic Azad University, firoozabad, Iran) *Correspondence author: Fax: Mohammad_Riyahin@Yahoo.com ABSTRACT In Retrograde reservoirs below dew point pressure we have two phases in reservoir (gas and Liquid phase). Initially this liquid is immobile, but as soon as the critical liquid saturation has been exceeded, the liquid can eventually flow toward the wellbore and it can produce therefore, We define three different Recovery Factor (Dry gas RF which is the total produced gas at surface per total initial gas in place, Wet gas RF is the total produced wet gas at surface per total initial gas in place and Condensate RF which is the total gas condensate produced at surface per the total produced gas at surface) and then using simulation, the effect of some parameters such as reservoir permeability, percentage of critical condensate s saturation, percentage of heavy compositions, different scenarios of production and finally changes of flow rate on different RF after six years of production can be analyzed. Keywords: Gas Condensate Reservoirs, Dry gas RF, Wet RF, Condensate RF, Simulation. INTRODUCTION Many gas condensate reservoirs experience significant productivity losses as the reservoir pressure drops below the dew point [10]. This productivity loss is related to the liquid dropout in the vicinity of the wellbore and the subsequent decline in gas rate which is in associated with the reduction in gas relative permeability. The decrease in well deliverability is attributed to condensate accumulation and water blocking. The liquid condensate continues to accumulate, occupying portions of the rock that would otherwise be available for gas flow, and thus impedes the flow of the gas until a critical liquid saturation is reached that is similar to the value for residual oil saturation. Once the critical liquid saturation is exceeded, both the condensate and gas flow towards the wellbore, but condensate continues to accrue until a steady state saturation is reached that is somewhat higher than the critical condensate saturation. Several techniques have been used to increase gas well deliverability after initial decline. There are some ways leading to increase in production of gas condensate reservoirs including [3]. changes of produced flow rate, choosing the best scenario of production in surface, changes in the percentages of formed condensates in reservoir because of the chemical injection, changes in reservoir permeability specially regions near the well because of the hydraulic fracturing and finally changes in the percentage of heavy hydrocarbon components because of the solvent injections. The percentage of critical condensate s saturation and heavy compositions of one of the Iranian gas condensate reservoirs can be changed using simulator and based on the defined scenarios for permeability, methods of production flow rate [1]. Percentage of critical condensate s saturation and then the effect of these changes on behavior (increase or decrease) of different RFs can be analyzed [9].

2 FLUID MODEL AND RESERVOIR BULK MODEL USED IN SIMULATION The volumetric model for condensate reservoir to do the simulation is a single well model[5]. This model is made by compositional simulator. The structure of model is radial (R=80*θ=1*Z=1). The grid block near the well is fine and farther the well gradually be large. Vertical thickness is 100 feet and number of grid blocks from wellbore to drainage radius is 80 cells. The producing well is designed vertically in the center of reservoir. The nearest cell from well is 0.1 feet wide. Skin factor is assumed zero furthermore, wellbore storage and water gas contact are not simulated. Reservoir porosity is 30%.Fluid reservoir model taken from a well in one of gas condensate fields in Iran. Primary reservoir pressure =7000 psi and Dew point pressure is 5000 psi. In this simulation Peng- Robinson Equation of State (PR-EOS) is used to calculate the liquid mole volume. RESULTS AND DISCUSSIONS The reservoir and the fluid models were simulated using some different scenarios. The complete list can be seen in Table 1. with the major scenarios are including gas plateau rate, reservoir fluid composition (C20+ composition), critical liquid saturation, absolute reservoir permeability, and the selection of rate scheme. As were mentioned, the objective was meant to examine the Reservoir Recovery Factor. The scenarios selected include the parameters that are predicted to influence Reservoir Recovery Factor. for this phenomenon is that, reservoir pressure drop is increased because of the increase in reservoir production, and then the condensation rate is increased in reservoir which leads to loss of the substantial amount of condensates in surface. Besides, condensate production in reservoir and their accumulation near the well may lead to decrease in gas relative permeability and then consequently decrease in production rate. Finally, all the mentioned factors result in decreasing the recovery factor of gas condensate reservoirs Investigation of the effect of changes of C 20+ mole fraction on Reservoir recovery Factor We use Table 1. (The second scenario) and change the C20+ mole fraction of the reservoir fluid and then the effect of changes of C20+ mole fraction on recovery factor after six years of production is investigated separately. Figure 2. shows that, increasing the mole percent of heavy components decreases the recovery factor because the viscosity of reservoir fluid increases and their movement decreases, therefore production rate is decreased and finally decrease in production rate decreases the recovery factor. If the reservoir gas is consist of heavy components, the injection of produced light gases to the reservoir is considered to be an operational and important way to make the reservoir gas more lighter, decrease the gas viscosity and improve their movement. Investigation of the effect of changes of critical liquid saturation on reservoir recovery factor Investigation of the effects of produced flow rate on Reservoir Recovery Factor As it is shown in Table 1. (The first scenario) three different produced flow rates, 2, 5 and 12 MMSCF/D are set and then we investigate the effects of changes of the flow rate on recovery factor after six years of production. Figure 1. shows that recovery factor is decreased by increasing reservoir production and the reason As it is shown in Table 1. (The third scenario), we change the critical liquid saturation and the effect of changes of the critical liquid saturation on recovery factor is investigated after six years of production. Figure 3. shows that recovery factor is increased with increasing the critical liquid saturation. Critical liquid saturation is defined as the maximum percentage of not moveable condensates in the reservoir. As it is

3 mentioned, condensates start forming in the reservoir when pressure drops below the dew point because of the production. When the percentages of these condensates are below the critical value, they are not moveable and production from the reservoir causes to increase in saturation percentage of condensates and they reach to the critical value in a special point, then they can move toward the production well [4]. Therefore, the movement of condensates in the reservoirs takes longer if the critical percentage of the condensates in the reservoir increases. So this phenomenon decreases the condensate production and consequently decreases in reservoir recovery factor. Investigation of the effect of changes in Absolute permeability on reservoir recovery factor As it is shown in Table 1. (The forth scenario), we change the reservoir absolute permeability and the effects of these changes on recovery factor after six years of production is investigated. Figure 4. shows that, increasing reservoir permeability increases the recovery factor because increasing the reservoir permeability makes fluid movement easier and therefore the reservoir production is increased. Investigation of the effect of gradual increase and decrease of gas production rate on reservoir recovery factor As it is shown in Table 1. (The fifth scenario) the effect of gradual increase and decrease in producing flow rate on recovery factor after six years of production is investigated. Figure 5. for six years of production shows that, gradual increase in production flow rate increases the recovery factor and gradual decrease in production flow rate decreases the recovery factor as well because gradual increase in production flow rate gradually increases the movement of the reservoir fluid near the well and this increases the sensitivity of the accumulated condensates near the well, therefore the condensates can easily move toward the production well. Increase in reservoir recovery factor and gas production is a result of decrease in accumulated condensates near the well. CONCLUSION Investigation of the effects of all five mentioned scenarios on reservoir recovery factor Table 2. shows the effects of all five simulated scenarios on condensate recovery factor, wet gas RF and dry gas RF. Results show that: The maximum effect of reservoir permeability changes from 2md to 15md is on dry gas RF and the minimum effect is on condensate RF. The maximum effect of condensate critical saturation changes from 0.2 to 0.5 is on dry gas RF and the minimum effect is on condensate RF. The maximum effect of C20+ mole fraction changes from 0.1 to 0.38 is on condensate RF and the minimum effect is on dry gas RF. The maximum effect of production flow rate changes from 2 MMSCF/D to 15 MMSCF/D is on dry gas RF and the minimum effect is on condensate gas RF. The maximum effect of gradual increase and decrease in production flow rate is on wet gas RF and the minimum effect is on dry gas RF. Well production rate, components of reservoir gas, percentage of critical condensate saturation, absolute permeability and production scenarios are those important factors affecting the Reservoir Recovery Factor in which absolute Permeability and percentage of Critical Liquid saturation are more important than the others. Choosing the standard and optimum flow rate is very important in increasing the production rate of gas condensate reservoirs. Gas injection of methane and other light components [8]. can act as a way for condensates to make the heavy components of the reservoir fluid soluble and consequently improving the production rate and recovery factor [2].

4 In the case of high critical liquid saturation of the reservoir, chemical injection such as methanol and some other solvents decreases the critical liquid saturation and increases the production from reservoir. Hydraulic fracturing in silicate formations and also acidizing in carbonate formations can be used to increase the reservoir permeability and production. Note that, hydraulic fracturing should not be used as the permanent solution because with pressure drop, accumulation of the condensates change place from wellbore to the regions near the fractures and this may block the fractures and reduces permeability and production. REFRENCES 1.Alavian, S. Ahmad and Whitson, C.H. (2005). CO2 IOR Potential in Naturally-Fractured Haft Kel Field, Iran, IPTC Alavian, S. Ahmad and Whitson, C.H. (2010). CO2 Modeling CO2 Injection Including Diffusion in a Fractured-Chalk Experiment. SPE ATCE. 3.Ayala H., L.F., and Ertekin, T. (2007). Neuro- Simulation Analysis of Pressure Maintenance Operations in Gas-Condensate Reservoirs, J. Pet. Science and Engineering, v. 58, n. 1-2, p Ayala, L.F., Eltohami, E., and Adewumi, M. (2002). Avoiding Pitfalls in Multiphase Thermohydrodynamic Coupling. Paper presented in the 25th Engineering Technology Conference on Energy of ASME, Houston. 5.Ayala H., L.F., Ertekin, T., and Adewumi, M. (2006). Compositional Modeling of Gas- Condensate Reservoirs in Multimechanistic Flow Domains. SPE Journal, v. 11, n. 4, p Craft, B. C., Hawkins, M. F. (1991). Applied Petroleum Reservoir Engineering, Second Edition. Prentice-Hall. Inc. New Jersey. U.S.A. 7.Danesh, A.(1998). PVT and Phase Behavior of Petroleum Reservoir Fluid. Elsevierscience B.V. 8.Farias, M., Ayala H., L. F., Watson, R. (2009). Experimental and Zero Dimensional Analysis of CO2-N2Gas Cyclic Injection Processes. Petroleum Science and Technology, v. 27:1. 9.Hoier, L., Cheng, N., and Whitson, C.H. (2004). Miscible Gas Injection in Undersaturated Gas-Oil Systems. SPE paper 90379presented at the Annual Technical Conference & Exhibition, Houston. 10.McCain, W. D. (1990). The Properties of Petroleum Fluid. PennWell Publishing Co.Tulsa, Oklahoma..

5 Table 2. Effects of all five simulated scenarios Table 1. Simulation Scenarios 1 Scenarios Gas plateau rate variation 2 Reservoir fluid variation Critical liquid saturation variation Absolute permeability variation Gas rate scheme variation Specific Case Plateau rate = 2 MMSCF/D Plateau rate = 5 MMSCF/D Plateau rate = 12 MMSCF/D C20+ mole fraction = 0.1 C20+ mole fraction = 0.11 C20+ mole fraction = 0.13 C20+ mole fraction = C20+ mole fraction = 0.2 C20+ mole fraction = 0.25 C20+ mole fraction = 0.31 C20+ mole fraction = 0.38 Critical liquid saturation = 0.5 Critical liquid saturation = 0.4 Critical liquid saturation = 0.3 Critical liquid saturation = 0.2 Permeability = 2 md Permeability = 4 md on recovery factors Permeability = 6 md Permeability = 8 md Permeability = 10 md Permeability = 15 md Plateau rate = 3.5 MMSCF/D Increasing rate gradually : 2; 6; 9; 12 MMSCF/D Decreasing rate gradually : 12; 9; 6; 2 MMSCF/D SCENARIOS PERMEABILIT Y (MD) CRITICAL LIQUID SATURATION MOLE FRACTION C20+ PLATEAU RATE (MMSCF/D) RATE SCHEME (MMSCF/D) SCENARIO S VARIATION 2MD 15MD TO CONDENSAT E RF (%) TO TO 0.38 WET GAS RF (%) DRY GAS RF (%) TO ,6,9,12 12,9,6,

6 Figure 1.Effects of produced flow rate on Reservoir Recovery Factor Figure 2. Effect of changes of C 20+ mole fraction on Reservoir recovery Factor

7 Figure 4.Effect of changes in Absolute Figure 3.Effect of changes of critical liquid permeability on reservoir recovery factor saturation on reservoir recovery factor

8 Figure 5.Effect of gradual increase and decrease of gas production rate on reservoir recovery factor